Various sensors for measuring the moisture content of a sample are known in the art. The various types of sensors are their basic principles of operation are described in a paper by Stanley Ronchinsky entitled "An Electrochemical Sensor for Trace Moisture in Gases," Moisture and Humidity, Measurement and Control in Science and Industry, 1985, pages 699 to 706. Of the different types of sensors described, the sensor type referred to as the "Electrolyzing Sensor" appears to be best suited for applications where continuous monitoring of moisture is required and for other industrial applications for the detection of trace amounts of moisture in samples.
Electrolytic sensors employ a moisture scavenger to trap the moisture in the sample and measure the amount of current required to electrolyze the trapped moisture. The operating principle of these sensors is Faraday's law of electrolysis in which the electrical charge required to electrolyze the water is the measure of the water content of the sample. Electrolysis is carried out by a pair or pairs of electrodes disposed in the body of the sensor.
The Keidel cell is an example of a moisture sensor operating on the electrolysis principle. Essentially the Keidel cell consists of a body which contains a pair of noble metal electrodes and which is packed with a suitable water scavenger such as phosphorous pentoxide. A sample stream is introduced into the body and the moisture is retained by the hygroscopic scavenger. The retained moisture is electrolyzed at the electrodes and the current required to electrolyze the moisture is measured. The Keidel cell and other forms of electrolytic sensors operating on the same principle require that the active area of the sensor be exposed to a constant flow of sample past the electrodes of the sensor. Variations in the sample flow rate can result in erroneous measurements and over time the hygroscopic scavenger tends to plug, causing a restriction in the flow of sample through the sensor, thus shortening the useful life of the sensor. An alternative to this is to introduce a known volume of sample into the sensor and measure the total current over time required to completely electrolyze the moisture in the known volume of sample. This is time consuming and does not lend itself to continuous monitoring. In addition, sensors of the type described have slow response times to changes in the moisture content of the sample and are not sensitive over a wide range of moisture content.
Accordingly, it would be highly desirable to provide a moisture sensor which retains the accuracy and reliability of the electrolytic sensors and which is sensitive over a wide range of moisture content. In addition, it would be desirable to provide a moisture sensor which measures the moisture content of a sample independent of the flow rate or volume of the sample to which the sensor is exposed, thus rendering the sensor more useful for the measurement of moisture contained in a solid as well as in a fluid.